In the oilfield industry, portable tanks have many uses, including receiving produced fluids from a hydrocarbon well. In this instance, it is known to produce the fluids directly into a first tank on site. Some separation of the initially produced fluids can occur in the first tank such that sand and heavier solids remain in the first tank while lighter hydrocarbons are transferred to a second tank located at the second site also. In this instance, each tank requires secondary containment which is commonly provided by a complex and time consuming assembly of a barrier or dike system about the tanks to support a membrane thereacross upon which the tanks are supported. The containment volumes of the membranes draped over the perimeter barriers are greater than the storage volumes of the tanks.
In some instances, it is known to provide a storage tank in a portable configuration on a skid base as disclosed in U.S. Pat. No. 5,392,911 by Gillispie et al. and U.S. Pat. No. 4,960,222 by Fields. Each tank requires its own integral containment having a containment volume at least the size of the tank such that the containment portion represents a considerable portion of the material and manufacturing cost of the overall tank structure. In the instance that any treatment is performed on site to separate some of the lighter hydrocarbons, a second tank assembly is required with its own containment structure.